It's a little "cup" like to the right (there are many). It is shaped to seal around your eye and wash out whatever is there that isn't supposed to be. You can use a special eye wash or salt-water mixture or even just CLEAN water, put your eye to the cup so that it's sealed. Lean back and blink a few times to let the water wash out your eye.
This is a necessary item in your first aid kit. Be sure to include several, and different varieties.